Titanium bi-alloy composites and manufacture thereof



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STANLEY ABKOWITZ ALBERT R. KAUFMANN &

BY ALAN K. WOLFF 6 51524414, W

ATTORNEYS Feb. 13, 1968 owrrz ETAL- 3,368,881

' TITANIUM BI-ALLOY COMPOSITES AND MANUFACTURE THEREOF Filed April 12, 1965 5 Sheets-Sheet 2 IBOIIIIIIIIIIIIIII Y-AHoy Ti-2O Cb-7.5 Al

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ALBERT R. KAUFMANN & BY ALAN K. WOLFF ATTORNEYS Mass.

Filed Apr. 12, 1965, Ser. No. 447,119 3 Claims. (Cl. 29192) ABSTRACT OF THE DISCLOSURE A composite of at least two titanium base alloy components, for example, two-thirds by weight of a Ti-6Al-4V alloy component, and one-third of a Ti-20Cb- 7.5Al alloy component, in which the Ti-6Al-4V alloy component is ductile at room temperature and drops off rapidly in strength as its temperature is increased above 900 F., in which the Ti-20Cb-7.5Al alloy component is brittle at room temperature but has high strength at 1200 F and in which the composite is characterized by having a desired combination of a high strength property at 1200 F. originating from one component and a ductile property originating from the other component.

The invention relates to titanium base bi-alloy composites mill products and their manufacture, and more particularly to composites of a plurality of different alloys of the same base metal wherein each alloy in the composite has one or more desired properties but is deficient at least in one other property exhibited by another alloy in the composite, thus providing a product having a 'desired combination of properties heretofore unknown or unobtainable in any known alloy of such base metal.

For example, developments and uses of titanium alloys and mill products for some time have been limited in attempts to obtain a titanium base metal mill product having ductility and also having high temperature strength at, say, 1200= F. equivalent to that presently obtainable in ductile titanium alloys at 900 F. The lack of such a product practically has imposed a 900 F.l000 F. elevated temperature limitation on the use of titanium alloys.

Known titanium alloys have desired ductility but are deficient in elevated temperature strength, such for example as the Ti-6Al-4V alloy which has desired ductility but its strength at 900 F. drops off rapidly as its temperature is increased above 900 F. Other known titanium alloys have desired strength characteristics at 1200 F. but are brittle.

Thus, a problem has existed in the art of how to overcome the 900 F.l000 F. elevated temperature limitation on the practical use of titanium alloys.

We have discovered a solution to this problem which eliminates the 900 F.-1000 F. limitation imposed on the use of titanium alloys and satisfies the existing need for a titanium base mill product which is ductile and has high temperature strength at, say, 1200 F. equivalent to that now obtained in ductile titanium alloys at 900 F. but whose strength drops off rapidly when heated above such temperature.

In accordance with the invention two or more different titanium alloys, each having one or more of the desired properties, are converted separately to high purity powder form and then are blended in the desired proportions to obtain the desired combination of properties. The mixed particles then are heated, compacted and consolidated by plastic deformation reduction and working to the necessary extent or degree to produce a wrought mill bi-alloy composite product. During the heating and hot working nited States Patent the temperature is controlled such as to avoid or minimize substantial or harmful diffusion of any one of the component alloys into another.

Another facet of the problem involves the necessity on the one hand of maintaining the identity of each alloy component of the composite (so as .to retain the desired property or properties of each component), and on the other hand of securing a necessary bond between the particles of each of the components without interditfusion or alloying of the components with one another.

This, in accordance with the invention, is accomplished by preparing the components of the composite to be made in but not limited to the manner set forth in the Kaufmann Patent No. 3,099,041 for Method and Apparatus for Making Powder. By these means and procedures suitably fine, uncontaminated particles of the component alloys may be prepared for mixing. Suitable interface bonding between the particles of the components cannot be obtained unless the particles are clean, that is free of contamination as by oxidation. On the'other hand preparation of the particles of the components by prior grinding or attrition procedures results in oxidation contamination which would prevent bonding the component particles without diffusion.

Accordingly, it is a fundamental object of the present invention to provide new titanium bi-alloy composites having a new combination of propertiesductility and strength at 1200 F. equivalent to that known in ductile titanium alloys only at 900 F., to thereby eliminate the present 900 F .l000 F. elevated temperature limitation existing in the practical use of titanium alloys.

Furthermore, it is an object of the present invention to provide a new procedure for making as wrought mill products the new titanium bi-alloy composites described.

Also, it is an object of the present invention to provide a new concept of a metal and metal composite formed of components which would alloy with one another if melted together or if diffusion were permitted, but in which no interalloying exists and the identities of the components in the composite are retained to the extent that desired properties or characteristics of each component are present in the composite.

Furthermore, it is an object of the present invention to provide new bi-alloy composites in which the degree in which a desired property is present in the composite may be predicted reasonably in accordance with the arithmetic proportion of the component from which the property is derived is present in the composite.

Finally, it is an object of the present invention to provide new bi-alloy composites and procedures for making such products which have the indicated advantages, characteristics, properties, etc., which solve problems and satisfy needs existing in the art, which provide a new and heretofore unknown combination of properties, and which eliminate difiiculties heretofore encountered in the art and obtain the new results indicated.

These and other objects and advantages apparent to those skilled in the art from the following description and claims may be obtained, the stated results achieved, and the described difficulties overcome by the concepts, dis-- coveries, principles, procedures, methods, steps, compositions, composites, products and combined properties which comprise the present invention, the nature of which is set forth in the following general statements, preferred embodiments of whichillustrative of the best modes in which applicants have contemplated applying the principles-are set forth in the following description and illustrated in the drawings, and which are particularly and distinctly pointed out and set forth in the appended claims forming part hereof.

The nature of the concepts and discoveries of the present invention relating to wrought titanium base mill products may be stated in general terms as comprising a wrought titanium base bi-alloy composite mill product having ductility of about 9% room temperature elongation and 0.2% offset yield strength of about 50,000 p.s.i. and ultimate strength of about 69,000 p.s.i. at 1200 F. and containing one alloy component which is brittle at room temperature and another which has 60,000 p.s.i. yield strength at 1000 F. and only 25,000 p.s.i. yield strength at 1200 F.

The nature of the concepts and discoveries of the present invention from another aspect relating to 'bi-alloy composites may be stated in general terms as comprising a composite formed by a plurality of different alloys of the same base metal which upon heating together to melting or diffusion temperature will alloy with or diffuse into one another; which are bonded together but free of diffusion to an extent that characteristic properties of one or more of the component alloys deficient in at least one other alloy component are present in the composite, to

the extent that each component alloy retains its identity in the composite, and to the extent that the composite is characterized by absence of deficient properties of its components; and in which the strength of the composite may be calculated as being at least equal to the arithmetic means of the relative volume percents of the components involved.

The nature of the concepts and discoveries of the present invention relating to procedures for manufacturing .bi-alloy composites of the characteristic described may be stated in general terms as including the steps of providing commercially pure alloys of the same metal which alloys are to form components of the composite desired; separately converting said commercially pure alloys to suitably fine uncontaminated particle form in the manner set forth in Patent No. 3,099,041; blending the separate alloy particles in the desired proportions to obtain the desired properties in the composite to be formed; heating, compacting and consolidating by plastic deformation, reduction and working the uncontaminated blended particles preferably at least to to 1 area reduction to produce a ductile wrought mill bi-alloy composite product; and controlling the temperature of the alloy components during heating and hot working to a temperature 'below that at which harmful diffusion of the components with one another can occur.

By way of example, characteristics of the improved bialloy composites of the present invention are shown in the accompanying drawings forming part hereof in which:

FIGURES 1 and 2 are graphs showing the room and elevated temperature tensile properties of one example of a bi-alloy composite of the present invention as well as comparative properties of the component alloys; and

FIGS. 3, 4, 5, and 6 are reproductions of a photograph and photomicrographs of the new bi-alloy composite.

Similar numerals refer to similar parts throughout the various figures of the drawings.

In accordance with the invention and illustrative of the new concepts, two titanium alloys are employed as components of the resultant bi-alloy composite formed, in which the two alloys are compositionally and physically similar but represent two extremes of mechanical properties as a function of temperature. For example the alloys may be a Ti-6Al-4V alloy and a Ti-Cb-7.5Al alloy.

The term Ti-6Al-4V is an accepted mode of expression in the art identifying a titanium base alloy containing 6% aluminum and 4% vanadium by weight. Similarly, the term Ti-20Cb-7.5Al identifies a titanium base alloy containing 20% columbium and 7.5% aluminum by weight. Similarly, the term Ti-8Al-1Mo-1V used below identifies a titanium base alloy containing 8% aluminum, 1% molybdenum and 1% vanadium by weight.

The Ti-6Al-4V alloy is a ductile titanium alloy but its strength, though about 75,000 p.s.i. yield strength at 900 F., drops off rapidly as temperature is increased (FIG. 2). This alloy, which may be termed the X alloy, is intended to serve as the matrix of the composite.

The Ti-20Cb-7.5Al alloy is a high temperature, high strength alloy having limited low temperature formability and ductility. It may be termed the Y alloy and supplies the high temperature strength to the composite.

The X and Y alloys in commercial rod or ingot form, such as 1%" rods of the X-alloy and 3" ingots of triple consumable arc melted Y-alloy material, are machined to form consumable electrodes for the manufacture of high purity or uncontaminated powders or particles of the X and Y alloys in the manner set forth in Patent No. 3,099,041. The high purity particles so produced may be termed fine, shot particles. These shot particles of the X and Y alloys are then blended in the desired ratio, for example, two parts of the X-alloy to one part of the Y- alloy by weight. Based on density measurements, this represents a volume percent of about 68% Ti-6Al-4V and 32% Ti-20Cb-7.5Al.

The blended shot particles are then compacted in a mild steel can, which is evacuated in a usual manner at 900 F. and then the can and compacted blended material therein is extruded through a 10:1 reduction ratio at about 1600 F. to produce, say, a /2 rod of the composite material.

Metallographic examination indicates that both phases or alloys undergo (see FIGS. .3 and 4 for lateral and longitudinal photomicrographs at 50X) good uniform reduction under the described fabrication conditions and resulted in a fully densified rod with well bonded interfaces between the particles of the two component alloys with no harmful interparticle diffusion.

The importance of the preparation of the particles or powders of the alloy components by the shotting procedures indicated, is again stressed. This enables high purity powder to be provided. Oxide coatings on the particle surfaces of either the X or Y alloys would cause poor interparticle bonds.

Further, since each of the X and Y alloys are exceptionally sluggish in diffusion reaction at temperatures as high as 1600 F., the lack of any harmful diffusion when hot working the blended particles at 1600 F. to obtain the interface bond is evident.

It is believed that a number of factors contribute to the formation of a strong, nonbrittle interface bond between the particles of the component X and Y alloys. The alloys are compatible, being alloys of the same base metaltitaniumand each including aluminum as an alloying element. In addition these metal alloys permit a metal-to-metal bond to occur. Next, the similarity of the alloy systems seems to contribute to the high degree of physical compatibility between the two alloys. Also, the particular compositions of the Xand Y alloys minimize the possibility of any brittle interface formation or any localized embrittlement in the matrix of either component alloy due to diffusion.

At this point, comment is warranted regarding the use of the term composite herein. This term has been used in the prior art in connection with combining a metal and a ceramic, or a metal and a compound, or two metals which do not alloy one with another. Such prior art composites are of a different character than the composite characterizing the concept of the invention.

In the present instance the component X and Y alloys if melted together or if permitted to diffuse would form a new alloy such as a Ti-Al-Cb-V alloy which would not have any outstanding properties. In such resultant alloy the identity of the component X and Y alloys of titanium would be lost. That is to say, that the composites of the present invention are composites of alloys of the same base metal in which the identity and favorable properties of each component survive in the composite.

As stated, this is achieved in part by hot working at a temperature lower than the conventional Working temperatures for titanium and its alloys in order to minimize difiusion. For example, the normal extrusion temperature for titanium is about 1800 F., yet in preparing the bialloy composites of the present invention, an extrusion temperature of about 1600 F. is used.

The longitudinal tensile test samples from which the photomicrographs (FIGS. 3, 4 and 5) were made, were annealed at 1500 F. to insure the removal of any residual cold work, at least in the X-alloy component. In FIGS. 3, 4 and 5 the white areas are the X-alloy ductile material component and the dark areas are the Y-alloy high temperature strength material component of the composite product.

The tensile samples were tested at room temperature, 1000 F. and 1200 F. The results are tabulated below in Table I which also lists representative tensile values for the individual components. Also shown in Table I are calculated values based on an arithmetic mean for relative volume percents of the two components. Strength data also is shown graphically in FIGS. 1 and 2.

TABLE I any discrete interface between the components, indicating that a small, though not harmful, amount of diffusion had occurred and that a good metallurgical bond was achieved.

During elevated temperature testing, several small cracks were generated in the surfaces of the tensiles as shown in FIG. 6 which is taken at five magnifications. Subsequent sectioning did not reveal any sub-surface cracking. The fact that ,these cracks shown in FIG. 6 did not propagate to cause rapid failure is believed to be due to the presence of the X-alloy which acted as a crackarrestor. This appears to be confirmed by the showing of FIG. 5 (250 magnifications). The section shown is slightly below the surface of the tensile sample and indicates that the crack (the black transverse pod shape) initiated in the Y-alloy, the darker component, with its progress being arrested in the more plastic adjacent X-alloy material. Such resistance to crack propagation suggests the possibility of improved impact strength for the bi-alloy composite.

Tensile properties of bi-alloy composite 68% Ti-6A1-4V plus 32% Ti-200b-7.5A1by volume Property Test Temp. T1-6Al-4V Ti-Cb-7.5Al Bi-Alloy Calculated X-Alloy Y-Alloy Composite Values Ultimate Tensile Strength (p.s.i.) Room Temp--- 135, 000 177,000 155, 600 145, 000 1,000 F 75, 000 144, 000 107, 500 97, 000 1,200 F 40, 000 136, 000 68, 200 71, 000 0.2% Offset Yield Stress (p.s.i.) Room Temp- 125, 000 177, 000 147, 100 137, 700 60, 000 132, 000 91, 500 83, 000 l,200 F 30, 000 124, 000 49, 800 60, 000 Percent Elong Room Temp 15 0-2 9. 1 10 1,000 F 6 7. 7 22 1,200 F 30 11 16. 2 24 Analyzing these data indicates that the bi-alloy composite not only has developed strengths equal to the arithmetic mean of the components involved but at lower temperatures actually has exceeded calculated values. This discovery is important since it indicates that approximate strength values of composites of the character comprehended may be calculated in advance in order to determine the proportions of the materials to be blended to achieve particular desired strength levels.

The explanation of why the actual'strength values exceed the calculated values is not entirely clear. It is possible that the preparation of the particles by the shotting procedure indicated to produce high purity uncontaminated material may have a beneficial effect on the strength of the individual component alloys. If such is the case calculated values based on properties of material produced by common processing procedures would be too low.

The fact that the calculated values are slightly higher than actual values for the bi-alloy composite at 1200' F. is probably due, at least in part, to the use of excessively high values for the strength of the Ti-6Al-4V alloy at 1200 F. There is no known data available for this alloy at this temperature, the values being extrapolated as shown by the dotted line portions of the graphs for the X-alloy in FIGS. 1 and 2.

Regardless of the explanation it is clear that substantial gains have been made over the tensile properties of the X-alloy while maintaining reasonable room temperature ductility and adequate formability at moderate temperatures.

Metallographic examination of the fractured tensile samples revealed that the two components were well blended as shown in the transverse and longitudinal sections of FIGS. 3 and 4, respectively. It is of special interest to note in FIG. 4 that the Y-alloy (Ti-20Cb-7.5Al), the darker component, underwent a substantial amount of deformation during extrusion despite its greater stifiness. Examination at high magnifications did not reveal for the particular alloys discussed. For example, the Ti-8Al-lMo-1V alloy may replace the Ti-6Al-4V alloy as the X-alloy for forming the ductile matrix component of the composite to be made. The Ti-8Al-1Mo-1V has a higher high temperature operating range and should impart slightly higher high temperature strengths to the composite. The ductile component of the composite it is believed should always be present in a substantial amount to insure reasonable ductility. Thus, it is desirable that its mechanical properties, with respect to temperature, should be maximized to achieve a composite having the highest high temperature strength or high strength at the highest possible temperature.

In selecting a high temperature alloy for the Y-alloy, requirements for compatibility with the X-alloy used are important. Thus, the Y-alloy should have good high temperature strength and stability at temperatures up to 1200 F. It should have alloying elements which do not form brittle intermetallios with the X-alloy used. It should have substitutional alpha stabilizing additions which do not exceed the thermal instability level of either component. It should have a density which does not greatly exceed that of titanium and thermal coefficients which do not differ substantially from those of the X-alloy used. It must have some degree of castability, machineability or other type of formability which permits fabrication into an electrode for preparing the shot particles. It must have a minimal degree of room temperature ductility at least sufiicient to prevent premature failure under stress. The

Furthermore, other titanium alloys may be substituted Ti-20Cb-7.5Al alloy satisfies all of these requirements for use in making a bi-alloy composite with the Ti-8Al-lMo-1V alloy.

Also the composite to be formed need not necessarily be limited to two alloy components since a plurality of alloys of the same base metal, each of which have one or more properties desired to be established in the composite to supply or mask a deficiency in such property in the other components, are contemplated. Thus, in using the term bi-alloy composite herein, the term is intended to include a composite containing more than one component and is not limited to a composite containing only two components. The essential is that the composite provide a combination of properties derived from the components not present in combination in any one component nor in an alloy or diffusion of the components.

Finally the concept includes bi-alloy composites of a plurality of alloys of the same base metal without being limited necessarily to alloys of titanium, since the principles of the invention can be realized with alloys of other base metals.

The new products and procedures of the invention, accordingly provide for the manufacture of titanium base mill products having a combination of properties heretofore unknown in the art; provide for overcoming the existing 900 FH1000 F. elevated temperature limitation on the practical use of titanium alloys; provide new composite products having the advantages, characteristics, properties, combination of properties and uses indicated; solve problems and satisfy needs existing in the art; eliminate difliculties heretofore encountered in the art; and obtain the new results described in a simple manner.

In the foregoing description certain terms have been used for brevity, clearness and understanding; but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such terms are used for descriptive purposes herein and are intended to be broadly construed.

Moreover, the description and illustration of the invention and the new procedure and products are by way of example and the scope of the invention is not limited to exact details described, because various products may be made without departing from the fundamental concepts and principles of the invention.

Having now described the features, concepts, discoveries and principles of the invention, the characteristics,

properties and manufacture of the new bi-alloy composite wrought mill products, and the advantageous, new and useful results obtained; the new concepts, discoveries, principles, procedures, methods, steps, compositions, composites, products and combined products characterizing properties, and mechanical equivalents obvious to those skilled in the art are set forth in the appended claims.

We claim:

1. A wrought mill product composite consisting twothirds by weight of a Ti-6Al-4V alloy component, and one-third of a Ii-20Cb-7.5Al alloy components; in which said alloy components comprise blended, high-purity, shot-formed particles consolidated by plastic deformation with a metallurgical interparticle bond; in which the composite has ductility of about 9% room temperature elongation, and 0.2% offset yield strength of about 50,000 p.s.i. and ultimate strength of about 69,000 p.s.i. at 1200 F.; in which the Ti-6Al-4V alloy component has only 25,000 p.s.i. yield strength at 1200 F.; and in which the Ti-20Cb-7.5Al alloy component is brittle at room temperature.

2. A wrought mill product composite consisting twothirds by weight of a titanium base alloy first component selected from the group consisting of a Ti-6Al-4V alloy and a Ti-8Al-lMo-1V alloy, and one-third of a Ti-20Cb- 7.5Al alloy second component; in which said alloy components comprise blended, high-purity, shot-formed particles consolidated by plastic deformation with a metallurgical interparticle bond.

3. A wrought mill product composite as defined in claim 2 in which the first component is ductile at room temperature and has only 25,000 p.s.i. yield strength at 1200 F.; in which the second component is brittle at room temperature; and in which the mill product composite has ductility of about 9% room temperature elongation, and 0.2% offset yield strength of about 50,000 p.s.i. and ultimate strength of about 69,000 p.s.i. at 1200 F.

References Cited UNITED STATES PATENTS 3,220,808 11/1965 Davies 29l98 3,235,346 2/1966 Hucke 29l9l.2 3,293,006 12/1966 Bartz 29182 HY LAND BIZOT, Primary Examiner. 

